Process for the asymmetric synthesis of chiral α-hydroxy-2-nitrobenzenepropanoic acid

ABSTRACT

Disclosed herein is a process for producing an asymmetric indoline-2-carboxylic acid of the structural Formula: ##STR1## wherein X is hydrogen, bromine, chlorine, C 1-4  alkyl or C 1-4  alkoxy, which comprises: 
     (a) assymetrically reducing an o-nitrophenylpyruvic acid III by contacting the acid III with a reducing complex formed from (R)-proline or (S)-proline, respectively, and sodium borohydride in an inert solvent to form, respectively, an (S) or (R)-α-hydroxy-2-nitrobenzenepropanoic acid IV; 
     (b) reacting, respectively, said (S) or (R)-α-hydroxy-2-nitrobenzenepropanoic acid III with a Vilsmeier chlorinating reagent in which the chlorinating agent thereof is selected from a group consisting of thionyl chloride, oxalyl chloride, phosphorus oxychloride, phosphorus pentachloride and sulfuryl chloride and the amide thereof is selected from a group consisting of dimethylformamide, diethylformamide, dimethylacetamide and diethylacetamide, said reaction being run at temperatures of at least 20° C., in order to obtain, respectively, and (R) or (S)-α-chloro-2-nitrobenzenepropanoic acid IV; 
     (c) reducing the nitro group of said (R) or (S)-α-chloro-2-nitrobenzenepropanoic acid (V) to an amino group; and 
     (d) cyclizing the resulting (R) or (S)-α-chloro-2-aminobenzenepropanoic acid in aqueous base.

This application is a divisional application of co-pending applicationSer. No. 829,662, filed on Feb. 14, 1986 (now U.S. Pat. No. 4,644,081,issued Feb. 17, 1987); which, in turn, is a divisional application ofco-pending application Ser. No. 700,379, filed on Feb. 11, 1985 (nowU.S. Pat. No. 4,614,806, issued Sep. 30, 1986); which, in turn, is in acontinuing application of co-pending application Ser. No. 565,084, filedon Dec. 23, 1983 (abandoned).

The invention relates to a novel process for the asymmetric synthesis ofindoline-2-carboxylic acids of the structural Formulas I and II, whereinX can be hydrogen, chlorine, bromine, C₁₋₄ alkyl or C₁₋₄ alkoxy.##STR2##

The overall process is illustrated by the following diagram for thepreparation of (S) -indoline-2-carboxylic acid: ##STR3## Step A in thisprocess is also novel and patent protection is sought for it as well asfor the overall process. In addition the intermediate (R) or(S)-α-chloro-2-aminobenzenepropanoic acids (V) are novel and patentprotection is likewise sought for these compounds.

BACKGROUND OF THE INVENTION

Indoline-2-carboxylic acids have heretofore been prepared by firstforming the corresponding indole-2-carboxylic acid and then reducing the2,3-double bond. The chief drawback in such preparations has been thelack of a satisfactory process for reducing the 2,3-double bond.

One of the methods of obtaining the indole-2-carboxylic acid to bereduced has been via the Reissert synthesis wherein o-nitrophenylpyruvicacid (III above where X is hydrogen) is reductively cyclized directly tothe indoline-2-carboxylic acid using zinc and acetic acid or ferroussulfate and ammonium hydroxide. See Weissberger, ed., The Chemistry ofHeterocyclic Compounds. Vol. 25, part 1, pp. 396-399 (WileyInterscience, New York, 1972).

Three methods are reported for reducing indole-2-carboxylic acid toindoline-2-carboxylic acid. Hudson and Robertson, Australian Journal ofChemistry, 20, 1935-41 (1967), first converted the acid to the amide andreduced the 2,3-double bond of the amide using phosphoniumiodide/hydriodic acid. They then converted the resultingindoline-2-carboxamide to the desired indoline-2-carboxylic acid byhydrolysis. Y. Omote et al., Nippon Kagaku Zasshi, 87, 760 (1966), alsoreported an indirect reduction. [See Stanton et al., Journal ofMedicinal Chemistry, 26, 1267-77 at 1268 (1983)]. In this method theindole-2-carboxylic acid was first converted to the N-acetyl derivativewhich was reduced by hydrogenation at atmospheric pressure in thepresence of platinum oxide. The resulting N-acetyl-indoline-2-carboxylicacid was then hydrolized to remove the acetyl group. Corey et al.,Journal of the American Chemical Society, 92, 2476-2488, at 2480 (1970),directly reduced indole-2-carboxylic acid ethyl ester using excess dryhydrogen chloride gas and tin and absolute ethanol in a sealed bomb.

After obtaining indoline-2-carboxylic acid by one of these procedures, aresolution process would then have to be performed in order to obtainthe desired chiral indoline-2-carboxylic acid. Here also one of thereported resolutions is indirect in that the indoline-2-carboxylic acidis first converted to its N-acetyl derivative. This derivative isresolved and the resolution product is converted back to theindoline-2-carboxylic acid. See Example 1 of U.S. Pat. No. 4,374,847 toN. Gruenfeld. A direct resolution of indoline-2-carboxylic acid using(+)-α-methylbenzylamine as a resolving agent is reported in TetrahedronLetters, Vol. 23, No. 16, pp. 1677-80, at 1678 (April/May, 1982).However, no detail on yields is given in this report.

Applicants' process differs from all prior processes for the preparationof I or II in that either I or II, as desired, may be obtained as asingle enantiomer without the concurrent formation of the opposingoptical antipode.

The classical resolution process, as described above, which involves thegeneration of both optical enantiomers followed by separation of thedesired from the undesired enantiomer, is less efficient because half ofthe material generated is lost in the form of the undesired enantiomer.The invention process is more efficient since it decreases the amount ofmaterial that is lost in the form of the undesired enantiomer.

The indoline-2-carboxylic acids of Formula I are useful as startingmaterials for the preparation ofN-(3-mercapto-2-alkyl-1-oxopropyl)-indoline-2-carboxylic acids andN-(2-substituted-1-oxoalkyl)-indoline-2-carboxylic acids thereof whichhave antihypertensive and angiotensin converting enzyme (ACE) inhibitoryproperties. These antihypertensive agents and ACE inhibitors aredisclosed in U.S. Pat. No. 4,303,583, issued on Dec. 1, 1981, to D. H.Kim and R. J. McCaully and in U.S. Pat. No. 4,350,633, issued on Sept.21, 1982, also to D. H. Kim and R. J. McCaully. Theindoline-2-carboxylic acids of structural Formulas I and II are usefulas intermediates in further asymmetric syntheses of a variety of α-aminoacids. (E. J. Corey, R. J. McCaully, H. S. Sachdev, J. Am. Chem. Soc.,1970, 92 2476).

DETAILED DESCRIPTION OF THE INVENTION

Applicants' process for producing an asymmetric indoline-2-carboxylicacid of the structural Formula: ##STR4## wherein X is hydrogen, bromine,chlorine, C₁₋₄ alkyl or C₁₋₄ alkoxy, comprises:

(a) assymetrically reducing an o-nitrophenylpyruvic acid of the Formula##STR5## wherein X is as defined above, by contacting the acid III witha reducing complex formed from (R)-proline or (S)-proline, respectively,and sodium borohydride in an inert solvent to form, respectively, an (S)or (R)-α-hydroxy-2-nitrobenzenepropanoic acid of the formula ##STR6##wherein X is as defined above;

(b) reacting, respectively, said (S) or(R)-α-hydroxy-2-nitrobenzenepropanoic acid IV wherein X is as definedabove, with a Vilsmeier chlorinating reagent in which the chlorinatingagent thereof is selected from a group consisting of thionyl chloride,oxalyl chloride, phosphorus oxychloride, phosphorus pentachloride andsulfuryl chloride and the amide thereof is selected from a groupconsisting of dimethylformamide, diethylformamide, dimethylacetamide anddiethylacetamide, said reaction being run at temperatures of at least20° C., in order to obtain, respectively, an (R) or(S)-α-chloro-2-nitrobenzenepropanoic acid of the formula: ##STR7##

(c) reducing the nitro group of said (S) or(S)-α-chloro-2-nitrobenzenepropanoic acid (V) to an amino group; and

(d) cyclizing the resulting (R) or (S) 2-amino-α-chlorobenzenepropanoicacid in aqueous base.

The process as illustrated for the synthesis of the (S)-enantiomer Ia(X═H) is shown above. A directly analogous scheme of reactions in whichall chiral centers are inverted would be used if the correspondingR-enantiomer (II) is desired. The asymmetric reduction ofo-nitrophenylpyruvic acid to (S)-α-hydroxy-2-nitrobenzenepropanoic acidutilizes a complex formed by contacting (R)-proline with sodiumborohydride in an inert organic solvent. The complex is generated bystirring the ingredients at 0° to 50° C. but preferably at roomtemperature for 1 to 24 hours but preferably for 18 hours. To theresulting dispersion is added dropwise a solution ofo-nitrophenylpyruvic acid in an inert ether solvent, such astetrahydrofuran (THF), dioxane, or dimethoxyethane, in a molar ratio ofcomplex to acid of 0.5 to 2.5 but preferably 0.85 to 1.0 and thereaction is allowed to proceed at 15° to 45° C., but preferably at 25°to 30° C. for one to ten days but preferably for 4 to 8 days. Thesolution was decanted from any insoluble residue and evaporated. Theresidues are digested with aqueous acid and extracted into an inert,immiscible organic solvent. Removal of the solvent affords primarily the(S)-α-hydroxy-2-nitrobenzenepropanoic acid which can be separated fromthe less soluble racemic compound by dissolution in an appropriate inertorganic solvent such as ether or dichloromethane. Removal of the solventaffords sufficiently pure (S)-α-hydroxy-2-nitrobenzenepropanoic acid tobe carried on to the next step. Alternatively, the intermediate can berecrystallized by treatment with an appropriate mixture of ethers andhydrocarbons or a single polar hydrocarbon solvent such as toluene.

In step B, the resulting (S)-α-hydroxy-2-nitrobenzenepropanoic acid (IV)is reacted with a Vilsmeier chlorinating reagent in order to form an(R)-α-chloro-2-nitrobenzenepropanoic acid (V) by replacement of thehydroxyl group with a chlorine atom. The Vilsmeier chlorinating reagentis formed from the combination of a chlorinating agent and an amide ofthe formula R₂ CONR₁ R₁, where R₁ and R₂ are preferably a methyl orethyl group. A suitable chlorinating agent may be selected from thionylchloride, oxalyl chloride, phosphorus oxychloride, phosphoruspentachloride, and sulfuryl chloride, with thionyl chloride beingespecially preferred. A suitable amide may be selected fromN,N-dimethylformamide, N,N-diethylformamide, N,N-dimethylacetamide, orN,N-diethylacetamide, with N,N-dimethylformamide (DMF) being especiallypreferred. The Vilsmeier reagent may be formed separately and then addedto the α-hydroxy-2-nitrobenzenepropanoic acid substrate, or theVilsmeier reagent may be formed in situ, usually by combining thesubstrate and the amide first and then adding the chlorinating agent.

In order to ensure chlorination at both the hydroxy and acyl sites ofthe substrate, thereby avoiding intermolecular condensation, the amideportion of the Vilsmeier reagent is preferably present in 2-6 moles permole of substrate. Good yields are obtained with 2-3 moles of amide permole of substrate. For the same reason, the chlorinating agent should bepresent in at least 2 moles per mole of substrate. Preferably thechlorinating agent is present in excess of 4 moles per mole of substrateand may be present in up to 34 moles per mole of substrate. Such largeexcesses are used particularly when no inert solvent is used for thereaction.

Accordingly, the reaction of step B can be run without a solvent or withan inert organic solvent such as dichloromethane, dichloroethane,chloroform, methylene chloride or ethylene chloride. Dichloromethane isthe preferred solvent. Additionally, temperatures above 20° C. arenecessary when reaction B is in progress. Preferably, the reactiontemperature is at least 25° C. and may vary up to about 50° C. dependingupon the particular Vilsmeier reagent and solvent used. A preferedtemperature range is 25°-35° C., and, at such temperatures, a reactiontime of 12-24 hours is preferred. Conveniently, an overnight reaction ofabout 18 hours at room temperatures of 25° C. is used. Evaporation ofsolvent and volatile components followed by quenching in ice affords theproduct which may be isolated by extraction into an inert immiscibleorganic solvent followed by evaporation. Purification of theα-chloroacid may be achieved by formation of a salt with an alicyclic orcyclic amine such as 2,6-dimethylpiperidine. The 2,6-dimethylpiperidinesalt is particularly useful for the purification of(R)-α-chloro-2-nitrobenzene-propanoic acid and its separation from minoramounts of racemic product that may have carried through the asymmetricreduction step.

The (R)-α-chloro-2-nitrobenzenepropanoic acid (V) or its amine salt isnext reduced in a basic medium to afford either the correspondingintermediate (R)-2-amino-α-chlorobenzenepropanoic acid (V) or thedesired (S)-indoline-2-carboxylic acid (I). The reduction may be donecatalytically in the presence of standard hydrogenation catalysts suchas palladium on carbon in a hydrogen atmosphere, in which case theIntermediate (V) is obtained. Alternatively, the reduction can becarried out with reducing agents that are compatible with a basicreaction medium, such as Raney nickel-hydrazine, in which case thedesired (S)-indoline-2-carboxylic acid (I) is obtained directly.

In those instances in which the intermediate (VI) is generated andisolation is preferred, it is most convenient to isolate the compound asa salt formed with an alicyclic or cyclic amine. 2,6-Dimethylpiperidineis the preferred amine. Conversion of (VI) to the(S)-indoline-2-carboxylic acid (I) is achieved by contacting adispersion of (VI) in a hydroxylic solvent with an alkaline metalhydroxide. Isolation of the product follows standard procedures.

As stated previously, if (R)-indoline-2-carboxylic acids (II) aredesired, the same process is used except that all the chiral centers,including that of proline in step A, are reversed.

X, as defined herein, may also include fluorine and "halogen" refers tofluorine, chlorine and bromine.

EXAMPLE 1 (+)-α-Hydroxy-2-Nitrobenzenepropanoic Acid

D-(+)-proline (7.50 g., 65 mmol) and sodium borohydride (2.46 g., 65mmol) were stirred gently together in dry tetrahydrofuran (400 ml.) for18 hours. To the resulting suspension was added dropwise a solution ofrecrystallized o-nitrophenylpyruvic acid (14.17 g., 67.7 mmol) in dryTHF (50 ml.). The resulting solution was stirred at room temperature forsix days, decanted, and evaporated. The resulting gum was stirred with10% aqueous HCl for five hours and the solution was extracted threetimes with 150 ml. portions of ether. The ether solution was dried(MgSO₄) and evaporated to provide the crude product as a gummy solid.Residual solvent was removed by pumping with an oil pump overnight togive 12.05 g. (85%) of product [α]_(D) ²⁵ +49.94° (c 2.46 EtOH).

The aqueous solution from which the product had been extracted wasstirred with 30 teaspoonfuls of Amberlite IR-120 (H⁺) ion exchange resinfor three hours, poured into a column, and the resin was drained andwashed with distilled water (1 L). The resin was then poured into abeaker and stirred for three hours with dilute ammonia (500 ml.). Theresin was again poured into a column, drained and washed with water (500ml.). The combined aqueous eluate was evaporated, and the solid residuewas recrystallized from methanol-acetone to provide 5.62 g. (74.9%) ofD-(+)-proline, m.p. 222°-224° C. [α]_(D) ²⁵ +83.99° (c 2.23, H₂ O).

EXAMPLE 2 (-)-α-Chloro-2-Nitrobenzenepropanoic Acid,2,6-Dimethylpiperidine Salt

To magnetically stirred N,N-dimethylformamide (23 ml., 297.3 mmol)cooled in ice was slowly added dropwise thionyl chloride (139 ml., ca.1.91 mol). A solution of (+)-α-hydroxy-2-nitrobenzenepropanoic acid[11.60 g., 54.9 mmol, [α]_(D) ²⁵ +49.94° (c 2.465, 95% EtOH)], obtainedfrom the chiral reduction of 2-nitro -α-oxobenzenepropanoic aciddescribed in Example 1, in dichloromethane (133 ml.) was then addeddropwise to the cold Vilsmeier reagent. The ice bath was removed and thesolution, protected from moisture, was left at room temperature (25° C.)for 18 hours. The resulting yellow solution was evaporated to syrup. Thesyrup was dissolved in dichloromethane and the solution was pouredcautiously onto stirred ice. The mixture was stirred for one hour duringwhich time it warmed to room temperature. The organic phase wasseparated, washed (×3) with water and dried (MgSO₄). Evaporation gave asyrup which was subjected to an oil pump vacuum for 0.5 hour. The syrupwas dissolved in ether and 2,6-dimethylpiperidine was added toapproximate neutrality (ca. pH 8 by pH paper). The crude titled compound(12.40 g., 66%) crystallized readily, m.p. 148°-153° C. Threecrystallizations (with decolorization) from methanol-ether gave 6.01 g.(32% of pure product, m.p. 164°-167° C. dec., [α]_(D) ²⁵ -41.49° (c1.005, 95% EtOH).

Analysis for: C₉ H₈ ClNO₂.C₇ H₁₅ N: Calculated: C, 56.06; H, 6.76; Cl,10.34; N, 8.17. Found: C, 56.17; H, 6.71; Cl, 10.17; N, 8.22.

EXAMPLE 3 (+)-α-Chloro-2-Aminobenzenepropanoic Acid,2,6-Dimethylpiperidine Salt

(-)-α-Chloro-2-nitrobenzenepropanoic acid, 2,6-dimethylpiperidine salt(2.06 g., 6.01 mmol) in anhydrous methanol (80 ml.) containing 2 drops(Pasteur pipette) of 2,6-dimethylpiperidine and 0.24 g. of 5% palladiumon carbon was hydrogenated at ambient temperature and pressure untiltheoretical uptake had been achieved. The mixture was filtered throughCelite, and the filtrate was evaporated. Coevaporation with cyclohexanegave a syrup which was subject briefly to an oil pump vacuum. Stirringwith ether gave 1.45 g. (77%) of a white crystalline solid, m.p.100°-101° C., [α]_(D) ²⁵ +16.00° (c 0.95, CHCl₃).

Analysis for: C₉ H₁₀ ClNO₂.C₇ H₁₅ N: Calculated: C, 61.43; H, 8.05; Cl,11.33; N, 8.95. Found: C, 61.31; H, 7.82; Cl, 11.53; N, 8.92.

EXAMPLE 4 (S)-(+)-2,3-Dihydro-1H-Indole-2-Carboxylic Acid

(+)-α-Chloro-2-aminobenzenepropanoic acid 2,6-dimethylpiperidine salt(1.25 g., 4.00 mmol) was added to magnetically stirred N sodiumhydroxide (4 ml.) under nitrogen. The solution was stirred undernitrogen overnight at room temperature. The pH was adjusted to a valuebetween 2 and 3 (pH paper) with conc. hydrochloric acid while cooling inice. The crystalline precipitate was collected and washed with severalaliquots of ice cold water. The product was dried briefly under vacuumover phosphorus pentoxide and then stirred with ether. The product wasdried over phosphorus pentoxide at 56° C. for several hours under oilpump vacuum to give 0.48 g. (74%) of the title compound, m.p. 155°-157°C., [α]_(D) ²⁵ +36.09° (c 0.87, DMF).

Analysis for: C₉ H₉ NO₂ : Calculated: C, 66.24; H, 5.56; N, 8.58. Found:C, 66.09; H, 5.67; N, 8.79.

IR, NMR and MS data were consistent with the structure.

EXAMPLE 5 (-)-α-Hydroxy-2-Nitrobenzenepropanoic Acid

(±)-α-Hydroxy-2-nitrobenzenepropanoic acid (37.0 g., 0.175 mol) wasadded to a warm solution of (-)-quinine (28.4 g., 0.0875 mol) in ethylacetate (350 ml.) and the mixture was swirled gently to effect solution.After standing for 36 hours the precipitated solid was filtered, washedconsecutively with ethyl acetate and ether. After drying there wasobtained the crude (-) acid (-) quinine salt (13.29 g.) - m.p. 165°-166°C. Recrystallization from ethyl acetate-ethanol (concentrated to about300 ml.) afforded the pure salt (11.55 g.) m.p. 167°-168° C.

This salt was partitioned between ethyl acetate and a small volume of10% aqueous HCl. The aqueous phase was extracted twice with ethylacetate, and the combined ethyl acetate extracts were washed with brine.The solvent was removed under vacuum and the residue was dissolved inether. The solution was filtered and again evaporated to dryness toprovide 4.11 g. of the product, [α]_(D) ²⁵ -59.40° (c 2, EtOH).

EXAMPLE 6 (+)-α-Chloro-2-Nitrobenzenepropanoic Acid,2,6-Dimethylpiperidine Salt From (-)-α-Hydroxy-2-NitrobenzenepropanoicAcid That Was Obtained Via Resolution With Quinine

To N,N-dimethylformamide (7.7 ml., 99.5 mmol) cooled in ice was slowlyadded thionyl chloride (46 ml., ca. 630 mmol) drop wise.(-)-α-Hydroxy-2-nitrobenzenepropanoic acid (3.83 g., 18.1 mmol),obtained in Example 5, was added in portions and the mixture was stirredat ice bath temperature for half an hour. The solution, protected frommoisture, was left at room temperature (25° or more) overnight). Thesolution was evaporated to a syrup which was dissolved indichloromethane. The dichloromethane solution was added cautiously tostirred ice, and the mixture was stirred for 1 hour, during which timeit warmed to room temperature. The organic layer was washed thrice withwater, dried over magnesium sulfate and evaporated to a syrup which wasbriefly subjected to an oil pump vacuum. The resulting syrup wasdissolved in ether, and 2,6-dimethylpiperidine was added until thesolution was slightly alkaline (pH paper). The crude titled product(4.87 g., 78% m.p. 145°-151° C.) crystallized readily. Fractionalcrystallization from methanol-ether gave 2.33 g. (37%) of the titledproduct [m.p. 164°-167° C., [α]_(D) ²⁵ +40.62° (c 0.965, 95% EtOH] asthe more insoluble component. (Racemic material was the more solublecomponent).

Analysis for: C₉ H₈ ClNO₄.C₇ H₁₅ N: Calculated: C, 56.06; H, 6.76; Cl,10.34; N, 8.17. Found: C, 55.82; H, 6.82; Cl, 10.13; N, 8.02.

EXAMPLE 7 (-)-α-Hydroxy-2-Nitrobenzenepropanoic Acid

L-(-)-proline (7.50 g., 65.1 mmol) and sodium borohydride (2.46 g., 65.1mmol) were stirred in tetrahydrofuran under nitrogen for sixteen hours.Recrystallized o-nitrophenylpyruvic acid (14.17 g., 76.5 mmol) intetrahydrofuran (50 ml.) was added dropwise, and the resulting solutionwas stirred at room temperature for six days. The solvent was evaporatedand the resulting gum was stirred with 10% aqueous HCl (200 ml.) for 4.5hours. A small amount of solid was filtered to provide nearly racemicα-hydroxy-2-nitrobenzenepropanoic acid, 1.25 g., m.p. 98°-102° C.,[α]_(D) ²⁵ -4.21° (c 2.045, EtOH).

The filtrate was extracted three times with ether (100 ml.), thecombined extracts were dried (MgSO₄) and evaporated (rotary evaporatorand then oil pump overnight) to provide 10.90 g. (76%) of the titlecompound, m.p. 45°-50° C., [α]_(D) ²⁵ -56.85° (c 2.065, EtOH).

The aqueous solution from which the product had been extracted withether was stirred with Amberlite IR-120 (30 teaspoonfuls) for 3 hoursand the resin filtered and washed with water (1 L). The resin was thenstirred with 50 ml. of concentrated NH₄ OH diluted to 500 ml. withwater. The resin was filtered, washed with 500 ml. of water and thewater removed under vacuum. The solid residue was dissolved in methanol(30 ml.), stirred and diluted by dropwise addition of acetone (300 ml.).The precipitated solid was filtered and dried to provide 3.75 g. (50%)L-(-)-proline, m.p. 228°-230° C. [α]_(D) ²⁵ -85.99° (c 2.22, H₂ O).

EXAMPLE 8 Purification of (-)-α-Hydroxy-2-Nitrobenzenepropanic Acid

34.01 g. Crude (-)-α-hydroxy-2-nitrobenzenepropanoic acid. [α]_(D) ²⁵-51.26° (c 1.67, 95% ethanol), was stirred magnetically indichloromethane (100 ml.). The resulting solid was collected on a filterand washed with dichloromethane (50 ml.). The solid [4.22 g., m.p.104°-108° C., [α]_(D) ²⁵ -9.56° (c 1.025, 95% ethanol)] was primarilyracemic material. Refrigeration of the filtrate (approximately 250 ml.)gave further crops [A, 1.07 g., m.p. 104°-107° C., [α]_(D) ²⁵ -19.33° (c1.19, 95% ethanol) and B, 0.54 g., m.p. 104°-197° C., [α]_(D) ²⁵ -16.54°(c, 1.215, 95% ethanol)] of primarily racemic material. Thedichloromethane solution was evaporated and the resulting syrupsubjected to an oil pump vacuum. The syrup solidified to give 26.50 g.of a yellow waxy solid , [α].sub. D²⁵ -63.04° (c 0.955, 95% ethanol).

Analysis for: C₉ H₉ NO₅ : Calculated: C, 51.19; H, 4.30; N, 6.63. Found:C, 51.47; H, 4.61; N, 6.53.

The above material (5.0 g.) was stirred magnetically with boiling ether(1 l.) and the mixture filtered through celite. The filtrate was boileddown to approximately 100 ml. and a small quantity of solid was removedby a further filtration. The resulting clear solution was boiled down toapproximately 50 ml. and poured into magnetically stirred pentane (400ml.). Scratching and stirring gave 3.06 g. of a yellow solid, m.p.63°-67° C., [α]_(D) ²⁵ -68.79° (c 0.99, 95% ethanol).

Analysis for: C₉ H₉ NO₅ : Calculated: C, 51.19; H, 4.30; N, 6.63. Found:C, 51.31: H, 4.34; N, 6.64.

The solid (2.98 g.) was dissolved in ether (25 ml.) and the solution wasdecolorized and evaporated to smaller volume. Toluene was added andproduct [0.62 g., m.p. 89°-91°, [α]_(D) ²⁵ -75.19° (c 1.31, 95%ethanol)] was allowed to crystallize slowly at room temperature in aflask open to the atmosphere.

Analysis for: C₉ H₉ NO₅ : Calculated: C, 51.19; H, 4.30; N, 6.63. Found:C, 51.54; H, 4.25; N, 6.94.

Recrystallization of 0.5 g. of the material from ether-toluene gave 0.35g. of product, m.p. 88°-90°, [α]_(D) ²⁵ -77.77° (c 1.705, 95% ethanol).

Analysis for: C₉ H₉ NO₅ : Calculated: C, 51.19; H, 4.30; N, 6.63. Found:C, 51.24; H, 4.22; N, 6.73.

IR, ¹ HNMR and mass spectral data were consistent for the abovestructure.

EXAMPLE 9 (+)-α-Chloro-2-Nitrobenzenepropanoic Acid,2,6-Dimethylpiperidine Salt

To magnetically stirred N,N-dimethylformamide (21 ml., 271.4 mmol.)cooled in ice was slowly added thionyl chloride (126 ml., ca. 1.73 mol)dropwise. A solution of 10.50 g. (49.7 mmol) of(-)-α-hydroxy-2-nitrobenzenepropanoic acid, obtained from the chiralreduction in Example 7, in dichloromethane (120 ml.) was added dropwiseto the cold Vilsmeier reagent. The ice bath was removed and thesolution, protected from moisture, was left at room temperature (25° C.)for 18.5 hours. The resulting yellow solution was evaporated to a syrup.The syrup was dissolved in dichloromethane and the solution was pouredcautiously onto stirred ice. The mixture was stirred for one hour duringwhich time it warmed to room temperature. The organic phase wasseparated, washed (×4) with water and dried (MgSO₄). Evaporation gave asyrup which was subjected to an oil pump vacuum for half an hour. Thesyrup was dissolved in ether and 2,6-dimethylpiperidine was added toapproximate neutrality (ca. pH 8 by pH paper). The crude titled compound(12.46 g., 73%, m.p. 148°-156° C. dec.) crystallized readily.Recrystallization from methanol-ether gave 7.74 g. (crop A) of product(m.p. 163°-166° C. dec.).

The mother liquor yielded further crops (B, 2.97 g., m.p. 140°-152° C.and C, 0.47 g., m.p. 139°-141° C.). Three crystallizations of crop Bfrom methanol-ether gave 0.60 g., m.p. 164°-167° C. dec. Recombinationof that material with the major crop A followed by recrystallizationgave 7.21 g. (42%) of pure product, m.p. 164°-167° dec. [α]_(D) ²⁵+41.03 (c 1.07, 95% EtOH).

Analysis for: C₉ H₈ ClNO₂.C₇ H₁₅ N: Calculated: C, 56.06; H, 6.76; Cl,10.34; N, 8.17. Found: C, 56.05; H, 6.73; Cl, 10.35; N, 8.14.

EXAMPLE 10 (+)-α-Chloro-2-Nitrobenzenepropanoic Acid,2,6-Dimethylpiperidine Salt

To magnetically stirred N,N-dimethylformamide (3 ml.) 38.8 mmol) cooledin ice was added slowly dropwise thionyl chloride (4.5 ml., 61.7 mmol).(-)-α-Hydroxy-2-nitrobenzenepropanoic acid (3.01 g., 14.2 mmoles),[α]_(D) ²⁵ -63.04° (c 0.955, 95% ethanol), in dichloromethane (35 ml.)was added to the cold Vilsmeier reagent, and the resulting solution,protected from moisture, was left at room temperature (25° C.)overnight. The solution was added to stirred ice, and the mixture wasstirred for 1 hour, during which time it warmed to room temperature. Thedichloromethane solution was separated and washed twice with water anddried (MgSO₄). Evaporation gave a syrup which was subjected to an oilpump vacuum for 0.5 hour. The syrup was dissolved in ether, and2,6-dimethylpiperidine was added to approximate neutrality (pH 8 with pHpaper). The crude titled product [2.95 g. (60%), m.p. 154°-158° C. dec.)crystallized readily. Fractional crystallization (methanol-ether) withdecolorization (Nuchar C-190N) gave 1.59 g. (33%) of titled product,m.p. 164°-167° dec., [α]_(D) ²⁴ +41.46 (c 1.275, 95% ethanol).

Analysis for: C₉ H₈ ClNO₄.C₇ H₁₅ N: Calculated: C, 56.06; H, 6.76; Cl,10.34; N, 8.17. Found: C, 56.42; H, 6.85; Cl, 10.25; N, 8.14.

IR, ¹ HNMR and mass spectral data were consistent with the abovestructure.

EXAMPLE 11 (+)-α-Chloro-2-Nitrobenzenepropanoic Acid2,6-Dimethylpiperidine Salt

To magnetically stirred N,N-dimethylformamide (3 mL) cooled in ice wasadded slowly dropwise thionyl chloride (4.5 ml., 61.7 mmol) and theresulting cold Vilsmeier reagent was diluted with dichloromethane (30ml.). (-)-α-Hydroxy-2-nitrobenzenepropanoic acid [3.00 g., 14.2 mmol,[α]_(D) ²⁵ -63.04° (c 0.955, 95% ethanol)] in dichloromethane (70 ml.)was added. The resulting solution, protected from moisture, was left atroom temperature (25° overnight. The solution was added to stirred iceand the mixture was stirred for 1 hour, during which time it warmed toroom temperature. The dichloromethane solution was separated and washedthrice with water and dried (MgSO₄). Evaporation gave a syrup which wassubjected to an oil pump vacuum for 1.5 hours. The syrup was dissolvedin ether and 2,6-dimethylpiperidine was added to approximate neutrality(pH 9 with pH paper). The crude titled product [2.98 g. (61%), m.p.155°-158° C. dec.] crystallized readily. Two crystallizations(methanol-ether) with decolorization (Nuchar C-190N) gave 1.91 g. (39%)of pure product; m.p. 164°-167° C. dec., [α]_(D) ²⁵ +42.11° (c 1.09, 95%ethanol).

Analysis for: C₉ H₈ ClNO₄.C₇ H₁₅ N: Calculated: C, 56.06; H, 6.76; Cl,10.34; N, 8.17. Found: C, 56.12; H, 6.71; Cl, 10.48; N, 8.18.

IR, ¹ HNMR and mass spectral data were consistent with the abovestructure.

EXAMPLE 12 (-)-α-Chloro-2-Aminobenzenepropanoic Acid,2,6-Dimethylpiperidine Salt

(+)-α-Chloro-2-nitrobenzenepropanoic acid, 2,6-dimethylpiperidine salt(2.06 g., 6.01 mmol) in anhydrous methanol (80 ml.) containing 2 drops(Pasteur pipette) of 2,6-dimethylpiperidine and 0.24 g. of 5% palladiumon carbon was hydrogenated at ambient temperature and pressure until thetheoretical uptake had been achieved. The mixture was filtered throughcelite and the filtrate was evaporated. Coevaporation with cyclohexanegave a syrup which was subjected briefly to an oil pump vacuum. Stirringwith ether containing a small proportion of cyclohexane gave 1.45 g.(77%) of product as a white crystalline solid, m.p. 101°-103° C.,[α]_(D) ²⁵ -15.73° (c 1.03, chloroform).

Analysis for: C₉ H₁₀ ClNO₂.C₇ H₁₅ N: Calculated: C, 61.43; H, 8.05; Cl,11.33; N, 8.95. Found: C, 60.89; H, 8.17; Cl, 11.60; N, 8.62.

EXAMPLE 13 (R)-(-)-2,3-Dihydro-1H-Indole-2-Carboxylic Acid

(-)-α-Chloro-2-aminobenzenepropanoic acid 2,6-dimethylpiperidine salt(1.25 g., 4.00 mmol) was added to magnetically stirred N NaOH (4 ml.)under nitrogen. The solution was stirred under nitrogen overnight atroom temperature. The pH was adjusted to a value between 2 and 3 withconcentrated hydrochloric acid and the mixture cooled in ice. Theresulting crystals were collected and washed with several aliquots ofice cold water. The product was dried briefly under vacuum overphosphorus pentoxide and then stirred with ether. The product wascollected and dried over phosphorus pentoxide at 56° for several hoursunder oil pump vacuum to give 0.43 g. (65%) of product, m.p. 151°-156°C., [α]_(D) ²⁵ -38.53° (c 0.95, DMF).

Analysis for: C₉ H₉ NO₂.0.1 H₂ O: Calculated: C, 65.52; H, 5.62; N,8.49. Found: C, 65.64; H, 5.64; N, 8.53.

What is claimed is:
 1. A process for producing an asymmetricα-hydroxy-2-nitrobenzenepropanoic acid of the structural Formula:##STR8## wherein X is hydrogen, bromine, chlorine, C₁₋₄ alkyl or C₁₋₄alkoxy, which comprises asymmetrically reducing o-nitrophenylpyruvicacid of the Formula: ##STR9## wherein X is as defined above, bycontacting the acid III with a reducing complex formed from (R)-prolineor (S)-proline, respectively, and sodium borohydride in an inertsolvent, and thereafter isolating the respective (S) or(R)-α-hydroxy-2-nitrobenzenepropanoic acid.